Human chorionic gonadotropin secretion and protein phosphorylation in chorionic tissue

The signal transduction system acts either monodirectionally (synergistically) or bidirectionally (antagonistically) in different tissues. We activated the signal transduction system in BeWo human choriocarcinoma cells and studied the effects on human chorionic gonadotropin (hCG) secretion, cell proliferation, and DNA synthesis. Protein phosphorylation in the cytosolic fraction was also studied by two-dimensional polyacrylamide gel electrophoresis. When phorbol 12-myristate 13-acetate (PMA) was added to cultures, hCG secretion was increased dose-dependently, but cell proliferation and 3H-thymidine uptake were not affected. When only cholera toxin was added, hCG secretion was also stimulated dose-dependently, but when both PMA and cholera toxin were added there was a synergistic potentiation of hCG production. In contrast, Ca ionophore A23187 had almost no effect on hCG secretion. Two-dimensional gel electrophoresis and autoradiography showed that phosphorylation of a 33 kd acidic protein was produced by cholera toxin, while phosphorylation of a 45 kd acidic protein and dephosphorylation of a 14 kd acidic protein were caused by the phorbol ester. These proteins may be specific substrates of protein kinase A and protein kinase C, respectively, in BeWo cells.